Active heat sink structure with flow augmenting rings and method for removing heat

ABSTRACT

An active heat sink is described for use in the transfer of heat from a heat generating device such as a semiconductor chip and the like with a heat sink having an embedded fan surrounded by a plurality of heat conducting flow augmenting rings separated by apertures through which a radially inward flow arises and with the rings being sufficiently axially separated to enable fan propeller tip vortices to penetrate the axial spacings so as to cause a substantial cooling of annular ring regions so as to raise the overall heat transfer coefficient of the active heat sink in a significant manner. Several embodiments are described.

PRIOR APPLICATION

This application claims the benefit of the filing date of ProvisionalApplication Ser. No. 60/012,098 filed Feb. 22, 1996 filed by Andrew I.Lemont all rights to which have been assigned to the same Assignee asfor this application.

FIELD OF THE INVENTION

This invention relates to heat sinks generally and more specifically toactive heat sinks incorporating a fan for causing an exchange of heatwith air flow.

BACKGROUND OF THE INVENTION

Active heat sinks which incorporate fans have been described in the artfor use in the cooling of heat generating devices such as electroniccomponents and the like. In U.S. Pat. No. 5,288,203 to Thomas a heatsink is shown and described wherein an axial fan is surrounded by aframe support formed by a heat transfer body having a pressuredifferential surface formed around the perimeter of the fan blades. Thepressure differential surface acts like a duct around the fan blades,and is so shown in some of the Figures. As stated the pressuredifferential surface provides a low pressure region and an axiallydisplaced high pressure region.

With particular reference to the embodiment shown in FIG. 11 of Thomas,a heat transfer body is shown including a number of vertically displacedrings attached in heat conducting relationship by studs with a circularfoundation. The rings are described to constitute an optimized heattransfer surface and their internal edges in effect form an air pressuredifferential surface. This is obtained by placing the rings in suchgeometric proximity to each other as to be able to form the air pressuredifferential surface. Ring spacings of the order of between 0.25 and 1millimeter are taught with 0.7 mm being preferred. The geometry or shapeof the rings are further so made as to enhance the axial pressuredifferential capacity.

The Thomas ring structure emphasizes the presence of a pressuredifferential surface and thus the need for the proximity of the Thomaspropeller tips to the inner edges of the surrounding rings so as tosimulate a duct through which air flow is produced by the fan. Thomasteaches the use of fine spacings between the rings and this bothdiminishes radial air flow therethrough and the extent to which tipvortices can penetrate into the axial ring spacings.

Although Thomas' tight ring spacings reduce induced radial air flow, hisfan structure appears to use a ring cage structure as taught by the U.S.Pat. No. 5,292,088 to H. E. Lemont since the operation of the Thomasdevice approaches a static air pressure characteristic as demonstratedwith curve 52 in FIG. 1A in the '088 patent. The emphasis in Thomas on aneed for a pressure differential surface requiring a tight ring-to-ringand propeller-to-ring spacings, indicates a failure by Thomas torecognize the significance of propeller tip vortices in the cooling of aheat sink.

In the US patents to Lemont and owned by the assignee of this invention,namely U.S. Pat. Nos. 5,292,088, 5,393,197 and 5,470,202, a ring cagestructure is described with a flow augmentation structure. With a ringcage structure as described in these patents the tip vortices from anaxial fan are converted to useful airflow and additional mass flowarises. This additional flow enters the spacings between the rings andjoins the mass flow from the fan. The '088 Lemont Patent teaches the useof a heat exchanger with fan cooling of rings.

Other patents showing a combination of a fan with a heat sink are U.S.Pat. Nos. 5,297,617 and 5,445,215, which show a fan in a duct to takeadvantage of turbulent air flow; U.S. Pat. No. 5,299,632 and 5,486,980and 5,494,098 which show a fan integrated with a fin type heat sink;U.S. Pat. No. 5,309,983 and U.S. Pat. No. 5,335,722, which teach a lowprofile integrated assembly of a fan with the fins of a heat sink forelectronic components; U.S. Pat. No. 5,353,863 for a pentium coolingdevice; U.S. Pat. No. 5,457,342 which in addition to the use of a fanshows a Peltier type cooling device; U.S. Pat. No. 5,475,564 which alsoshows a hold down device for connecting a heat sink to a CPU; U.S. Pat.No. 5,504,650 and 5,559,674 which illustrate a variety of differentcombinations of a heat sink and a cooling fan assembly; U.S. Pat. No.5,526,875 which shows the placement of a fan at the same level asvertically oriented fins of the heat sink; and U.S. Pat. No. 5,535,094,which shows a heat exchange device with a blower assembly and headers todirect the flow therethrough;

The cooling of heat sinks attached to semiconductor chips is becomingmore critical as semiconductors and CPUs such as the Pentium chipgenerate more heat, that typically is attributable to an increase inprocessing utilization or an increase in complexity. Generally,performance of such chips is affected by temperature increases andexhibit losses of functions of individual components as the operatingtemperature increases. It is, therefore, important to preventtemperature increases during operation of a semiconductor chip. Heatsinks, whether these operate without or with a fan (an active heat sink)are, therefore, rated for their thermal resistance, i.e. the amount oftemperature rise of the heat sink and thus also encountered in thesemiconductor device to which the heat sink is connected to, for eachwatt of dissipated power, expressed as °C./w. Thermal resistance is afunction of the volume and surface area in the heat sink so that anycomparison of thermal resistances of heat sinks should assume likevolumes or take into account any differences.

The effectiveness or heat transfer capability of an active heat sink isa function of the product of the surface area A, the temperaturedifference between the heat sink and the fluid (air) moving past theheat sink and a heat transfer coefficient H_(c). The heat transfercoefficient H_(c) in turn depends upon such factors as the geometry ofthe fluid flow and its velocity past the heat sink surfaces.

The prior art active heat sinks currently used in the industry generallyachieve the same best level of thermal resistance, that typically, forlike sized heat sink volumes, is in the range of about 1.4° C./w forPentium or 486 type chips. The thermal resistance level is to someextent dependent upon the speed of the fan used to remove heat from theheat sink, but by increasing the fan speed noise becomes anobjectionable side effect. Hence, fan speed cannot as a practical matterbe increased indefinitely. Fans also use power which increases at highfan speeds.

One limitation of prior art active heat sinks lies in the fact that thefan occupies precious space and its housing does not contribute to theheat transfer characteristic of the heat sink. Many concepts have beenintroduced to add surface area to the heat sink by making the housingthermally conductive and this has some benefits.

What is needed, therefore, is an active heat sink with which thermalresistance for an equivalent heat sink volume can be significantlyreduced in a manner that does not require more space, more electricalpower and does not introduce more fan noise.

SUMMARY OF THE INVENTION

With a heat sink in accordance with the invention a significantreduction of its thermal resistance is obtained. Thermal resistancelevels of the order of about 0.4° C./W and better for heat sinks fortypical semiconductor chips can be achieved without requiring more fanpower or speed, in an efficiently used space, and with low fan noise.

This is achieved with one embodiment in accordance with the invention byforming the heat sink with a plurality of parallel plates that aresupported by one or several heat conducting columns extending from abase plate. The parallel plates are shaped to form flow augmenting ringsaround a main air flow passage in which a fan is submerged so as topreferentially emphasize and utilize propeller tip vortices and enablethese to be incident on radially inward regions, preferably extendinginto the spacings between the flow augmenting rings for at least up toabout one quarter of the radius of the fan propeller. The base plate mayor may not have a plurality of fins as is conventional in heat sinks andwhen used these are located below the flow augmenting rings so that theair mass flow, including induced radial air flow, from the fan passesover and between the fins.

With an active heat sink in accordance with the invention theintegration of a fan such as described in the above identified Lemontpatents with a heat sink achieves an unexpected result in the cooling ofthe heat sink. The flow augmenting rings not only conduct heat from thebase plate but, because of a scrubbing action by the high velocity tipvortices of significant regions of the internal surfaces of the ringsand a relatively large and high velocity induced radial air flow acrossthe flow augmenting rings, an enhanced removal of heat from the heatsink is obtained to an unexpectedly high level. In addition, thedestruction of tip vortices provides an additional bonus in that noisefrom the fan is also significantly reduced.

In one embodiment in accordance with the invention the heat sink isformed of a single metal extrusion which incorporates a base plate, atleast one heat conducting column extending from the base plate and flowaugmenting plates supported by the heat conducting column. A main airflow passage then needs to be cut into the extrusion with appropriatemounting holes for a fan, i.e. the fan motor and fan blades, designed towork with the flow augmenting plates.

Alternatively a complex structure for a heat sink in accordance with theinvention can be formed of a casting or with suitable machining andintegrated with a fan to produce an effective low thermal resistanceactive heat sink.

The chordal dimension of the flow augmenting plates and their spacingsfrom each other along the fan rotational axis are selected to enhancethe scrubbing action of the inner regions of the flow augmenting platesby propeller tip vortices. This scrubbing action occurs at substantialgreater velocities than induced radial airflow so that the inner plateregions on which these vortices are incident exhibit heat transfercoefficients that are an order of magnitude greater than thoseattributable to other regions of the heat sink. By preferentiallyemphasizing and using tip vortices their contribution to heat removal isa major component in the overall heat transfer coefficient of the heatsink structure. Hence, the average overall heat transfer coefficientH_(c) of the heat sink can be at least 50 watts/m² /°C. and typically isin the range of 100 watts/m² /°C.

A conventional fan experiences a loss in air flow directly related tothe square of the pressure loss due to aerodynamic inefficiencies suchas from stall and friction. With a heat sink in accordance with theinvention, the additional heat transfer surface area provided by theflow augmenting rings is free from these types of losses. This resultsin a fan-heat sink thermal resistance that can be substantially belowconventional active heat sinks as well as below 0.4° C./W, instead ofabout 1.4° C./W, for typically sized heat sinks for semiconductor chipssuch as the Pentium or 486 chips.

Additional benefits of a heat sink in accordance with the invention areacoustic noise reductions due to the elimination of tip noise, a bearinglife improvement because the fan can operate at lower speeds to produceat least equivalent heat transfer of prior art devices, a low profiledesign with the fan submerged within the heat sink and an ease in themanufacturing by use of a number of different methods whether theseinvolve extrusions, castings or stamped assemblies.

Different configurations can be used with an active heat sink inaccordance with the invention. For example, the column used to place theflow augmenting plates in heat conducting relationship with the baseplate can be formed of thermoelectric cooling devices or a heat pipewhich improve the transfer of heat from the base plate to the flowaugmenting plates. The extrusion can be so shaped that either none or afew floating dies can be used. The flow augmenting rings or plates canbe directly supported from the base plate so as to enhance the removalof heat with an axial fan that is oriented to move air parallel to thebase plate. Bonded folded fins can be employed between the flowaugmenting plates to increase the heat sink surface area and provide aplate support structure without the use of columns. The flow augmentingplates can be made of a single folded strip of metal.

It is, therefore, an object of the invention to provide an apparatus andmethod for significantly improving the removal of heat from asemiconductor chip and other heat generating devices. It is a furtherobject of the invention to provide a method for the removal of heat froma heat generating device with an active heat sink while also reducingnoise, and improve bearing life of the fan.

These and other objects and advantages of the invention can beunderstood from the following detailed description of severalembodiments in accordance with the invention described in conjunctionwith the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent application contains two sheets of colorphotographs and two sheets of black and white photographs. Copies ofthis patent with color drawings will be provided by the Patent andTrademark Office upon request and payment of the necessary fee.

FIG. 1 is a perspective view of one heat sink in accordance with theinvention;

FIG. 2 is an end view of the heat sink as shown in FIG. 1;

FIG. 3 is a partial enlarged section view of the heat sink shown in FIG.1 taken along the line 3--3 in FIG. 2;

FIG. 4 is a section view of the heat sink taken along the line 4--4 inFIG. 2;

FIG. 5 is a section view taken along the line 5--5 in FIG. 4 of the heatsink of FIG. 1;

FIG. 6 is a section view taken along the line 6--6 of the heat sinkshown in FIG. 4;

FIG. 7A is a plot of performance curves of thermal resistance as afunction of fan air flow for a prior art heat sink and a heat sink inaccordance with the invention;

FIG. 7B is another plot of performance curves of temperature rises as afunction of power dissipated for a conventional heat sink and one inaccordance with the invention;

FIG. 7C is a thermal photo, in gray scale, illustrating in a side viewin elevation with colors the temperature and temperature gradients in anoperative active heat sink in accordance with the invention;

FIG. 7D is a thermal photo, in gray scale, of the same active heat sinkof FIG. 7C but taken at a about a 45° angle relative to the horizontalalong the same view as in FIG. 7C and resulting in a perspective thermalview of the heat sink;

FIGS. 7C' and 7D' are respectively identical photos shown in FIGS. 7Cand 7D but in color;

FIG. 8 is a perspective and exploded view of another heat sink inaccordance with the invention;

FIG. 9 is a section view of the heat sink shown in FIG. 8 and takenalong the line 9--9 in FIG. 8;

FIG. 10 is a similar section view as FIG. 9 for another but similar heatsink as shown in FIG. 8;

FIG. 11 is a similar section view as in FIG. 9 for a another but similarheat sink as shown in FIG. 8;

FIG. 12 is a side section view of a another heat sink particularlyinsensitive to axially adjacent obstacles in the path of the airflow;

FIGS. 13 and 14 are respectively side and perspective views of a fanused in the heat sink shown in FIG. 10;

FIG. 15 is a side view in partial section of another heat sink inaccordance with the invention wherein the flow augmenting rings areformed of a single multi-folded strip of material;

FIG. 16 is a top plan view of the heat sink of FIG. 15;

FIG. 17 is a side view in elevation of a heat sink in accordance withthe invention wherein the fan is supported in a vertical orientation;

FIG. 18 is a side view of the heat sink shown in FIG. 17 and taken alongthe axis of the fan;

FIG. 19 is a top plan view of a heat sink in accordance with theinvention wherein heat pipes are employed to convey heat from a baseplate to flow augmenting rings;

FIG. 20 is a section view taken along the line 20--20 in FIG. 19;

FIG. 21 is a front view of the heat sink of FIG. 19;

FIG. 22 is a perspective view of another heat sink of this inventionwith radially stepped flow augmenting rings for achieving a higher heattransfer coefficient; and

FIG. 23 is a side section view of the heat sink illustrated in FIG. 22with a fan.

FIG. 24 is a side view in partial section of another heat sink inaccordance with the invention wherein folded fins are employed betweenflow augmenting rings;

DETAILED DESCRIPTION OF THE DRAWINGS

With reference to FIGS. 1-6 a heat sink 30 in accordance with theinvention is shown. The heat sink 30 is formed of copper or an extrudedmaterial such as aluminum, though the invention is not limited to thistype of material and other materials and techniques for its manufacturecan be used such as stamped pieces that are brazed together or bycasting or with machining techniques. The heat sink 30 is designed foruse in the removal of heat from a semiconductor device 32 though its usecan be applied to other heat generating devices.

The heat sink 30 has a base plate 34 for heat conducting contact withthe outer surface of the chip device 32 and a plurality of parallelelongate fins 36.1-36.17 extending along the width of the heat sink. Thebase plate is shown as a flat plate though it is to be understood thatother shapes can be used such as a column coupled in heat conductingrelationship with the heat generating device. The base plate 34 haselongate straight columns 38 extending upwardly to support cross plates40 which form flow augmenting rings having the effects as shown anddescribed in the aforementioned Lemont patents. The flow augmentingplates 40 are axially separated from each other by spaces or air flowpumping apertures 41.

The use herein of numerals after a decimal point identifies specificitems while the numeral before the decimal point refers to the same typeof item in a more general way.

The columns 38 are elongate substantially solid plate walls, though theycould be perforated or discrete columns, which extend up from base plate34 and support the flow augmenting plates 40 to conduct heat to theplates 40. End located heat columns 38.1 and 38.4 provide additionalheat transfer capability though, because of their solidity, they closeoff portions of the radial ends of the air pumping spaces 41. A centralcylindrical cavity 42 is formed into the extruded heat sink 30 toreceive an axial fan 44 including its motor 46. The fan 44 has propellerblades 48 which extend towards the radially inward edges 50 of the flowaugmenting plates 40.

The heat sink 30 is so designed that its flow augmenting plates or rings40, their separations W or 41 and the clearances S between the rings andthe tips 52 of the propellers 46 provide the flow augmentation asdescribed in the aforementioned Lemont patents. Hence, the 5,292,088 and5,393,197 Lemont patents are incorporated herein by reference thereto,though it is to be understood from the following description thatvariations from the parameters and embodiments described in thesepatents are made to enable the heat sink of this invention to work in anoptimum manner, whether this is for performance or for cost reduction.

The effect of the flow augmentation plates 40 is to introduce a flowpattern as illustrated in FIGS. 3 and 5. This includes a main axial massflow indicated by arrows 56 and an induced additional useful mass flowindicated by arrows 58 obtained from a disturbance of the tip vortices59 from the propeller blades 46. The additional mass flow is directed ina radial direction between and over the flow augmenting plates 40. Thetip vortices 59 and this radial inward flow and a radial outwarddischarge flow have a significant velocity and as a result have adramatic impact on the cooling of the flow augmenting plates 40 and thusthe performance of the heat sink 30.

FIGS. 7C and 7D are thermal gray scale photographs of a heat sink inaccordance with the invention wherein the lighter grey or white areasrepresent a hotter region than the darker areas with gray levelstherebetween representing in-between temperatures. FIG. 7C' and 7D' areexactly the same photos but in color where blue represents coolerregions and deep red the hottest regions. The full scale of thetemperature range may be of the order of say 7° C. The particular heatsink is of the type as shown in FIGS. 1-6 without the middle columns38.2 and 38.3 or that as shown in FIG. 12 without the deflector on thefan. The heat sink in FIGS. 7C and 7D and 7C' and 7D' is subjected to aheat load of some 30 watts below the base plate.

Of particular interest is the appearance of annular cooled regions 61.1and 61.2 of and above the radially inner portions of the flow augmentingplates 40.1 and 40.2. These regions show a radial temperature gradientbecause they are primarily cooled by the impact of tip vortices such as59, see FIG. 3, and which are able to penetrate into the flow augmentingapertures 41.1 and 41.2 to air-scrub the radially inner plate regions.The view in FIGS. 7C and 7C' shows that the cooled regions 61.1 and 61.2penetrate the apertures 41 to the extent of about half the ring chordwidth at its widest point and at least the full ring chord at itssmallest width as shown at 73 in FIGS. 7D and 7D'. This penetration isabout 25% of the propeller radius R or main air flow passage radius R'.

The heat transfer coefficient in the tip-cooled ring regions 61,inclusive of contributions from induced radial flow, typically exceedsat least about 75 to about 100 W/m² /°C. In contrast, the heat transfercoefficient for other heat sink regions is predominantly determined byinduced radial air flow between parallel plates and is of the order ofabout 7 W/m² /°C due to a lower air velocity. This explains thesignificance of the tip vortices on the cooling of the heat sink asillustrated in FIGS. 7C and 7D.

FIG. 7A shows two curves 60 and 62 of thermal resistance of heat sinksmeasured in °C./Watt as a function of fan flow in CFM, cubic feet perminute. Curve 60 is demonstrative of the thermal resistance of a typicalprior art active heat sink wherein the best performance at 20 CFM isabout 1.4° C./W. Curve 62 shows a thermal resistance obtained with aheat sink 30 in accordance with the invention and of comparable size andsurface area as the prior art heat sink and fan and exhibits a thermalresistance of about 0.4° C./W, an improvement by a factor of more than3. It is expected that an even greater improvement in heat sinkperformance can be achieved.

The ability of an active heat sink to remove heat is a function of thesurface area, i.e. the total volume, exposed to the heat removing air,the temperature difference between the surface and air medium and a heattransfer coefficient H_(c). The heat transfer coefficient H_(c) is afunction of the velocity of the heat removing fluid, and thecharacteristics, such as surface roughness, smoothness or undulations ofadjacent ring surfaces and the geometry of the fluid flow. The higherthe velocity of the air flow the more heat can be removed and thus thelower the thermal resistance of the heat sink.

The improved reduction in thermal resistance of a heat sink inaccordance with the invention is primarily attributable to the combinedeffect of the radial air flows obtained over the flow augmenting ringswhich are in heat conducting relationship with the base plate 34 and thepenetration of propeller tip vortices into the pumping apertures 41between the rings 40. The radial flow represents a significant portionof the total mass flow, at least about 20% and frequently about 30%, andhas a sufficient velocity, typically about 300 feet per minute, sa as tocontribute to the overall high heat transfer coefficient of the heatsink structure. The tip vortices provide a dominant cooling effect byvirtue of their high velocities and ability to penetrate into thespacings between the rings 40. A heat transfer coefficient in the rangefrom about 50 and usually above about 75 and more than 100 watts/m² /°C.and more can be obtained.

FIG. 7B is illustrative of the effect of the heat sink 30 in accordancewith the invention. At 64 is a plot of the power dissipated and theresulting temperature rise typically encountered with a conventionalprior art active heat sink. At 66 is a similar plot for an active heatsink using flow augmenting rings and fan of the same size. Thetemperature rise is significantly lower with the heat sink of theinvention.

The discharge end of the main passage way 42 delivers the combined mainand induced air flows to the fins 36 where the back pressure forces airflow to exit through spaces 67 between the fins and between the fins anda column 38 and the available clearance 68 above the fins 36 and theflow augmenting ring 40.1. Some air is forced to discharge radiallyoutwardly through the aperture 41.1 between flow augmenting rings 40.1and 40.2 as illustrated with arrows 70 in FIGS. 1-3. The cooling effectof these air flows past the flow augmenting rings is dramatic asillustrated by the above described performance curves in FIGS. 7A and 7Band the thermal photos in FIGS. 7C, 7D.

The flow augmenting plates 40 are so placed with respect to the tips 52of the propeller blades that their tip vortices can brush the internalsurfaces of the flow augmenting rings with high velocity air flows andare sufficiently disturbed so that an induced radial flow can ensue.This flow is sufficient to enable an effective cooling of the rings 40.

The spacing S between the propeller blade tips 52 and the inner edges 50of the rings 40 is one of the factors used to accomplish this. Typicallythis gap S is in the range from about 2.0% to about 10% of the radius Rof the propeller blades for purposes of optimum flow enhancement, thoughwhen heat transfer is emphasized the gap S can be in the range fromabout 5% to about 15% of the radius R. The spacing S should not be solarge as to allow a reverse flow. Hence, for very large fans having aradius of the order of more than 30 inches the gap S tends to go below5% and when limited to about a half an inch tends to go to about 2%though care should be taken that the spacing S is not too small lest thetip vortices are substantially suppressed and the beneficial effect fromthe propeller tip vortices is not achieved.

The views in FIGS. 3 and 5 and other require that the spacing S appearsas varying in size from ring to ring. It should be understood, however,that in practice, the tips 52 are shaped so that their circle ofrevolution during rotation follows a cylindrical surface whereby thespacing S is effectively constant along the axial spans of the propellerblades 48.

The sizing and shaping of the flow augmenting rings 40 influences theradial air flows. Their preferred shape for flow augmentation iscylindrical, though that can not always be practical. As described andshown with reference to FIG. 16 in the Lemont '197 patent the rings canbe rectangular in sections and not necessarily annular in shape. In caseof other than annular shapes the pumping action from the apertures 41can still be obtained by staying within key parameters. Thus the flowaugmenting rings 40 can be of diverse shapes and also be effectivelysegmented as shown in FIGS. 1-6 where the outer columns 38.1 and 38.4prevent radial flow. Since the object of the rings is to provide heatdissipating surfaces, making them other than circular sections such asrectangular, provides more surface area and still effectively retain theheat sink function of the invention.

The flow augmenting rings are typically relatively thin structures whoseaverage radial width (outer radius less inner radius) or chord, C, andaxial spacings W are selected to enhance the flow augmentation. In caseof rectangular rings 40 as shown in FIG. 4 the ring chord dimension Cvaries around the perimeter and thus an average value (the largestradial width summed with the least width and then divided by two) isused. Generally if the ring chord C is too large the augmentation effectis diminished, though the larger surface area provides more heat removalability with the available turbulent type air flow through the adjacentaperture 41. If the ring chord dimension is made too small the flowaugmentation effect and the heat transfer capability are diminished.

The size of the ring chord C can be expressed as a function of the fanradius R or the main flow passage radius R', see FIG. 6, The radius R'is typically no more than from about 2.5% to about 15% larger than thefan radius R with 5% being typically used and with the lower rangeusually applicable to large diameter fans. Hence, C should be in therange from about 10 to about 50% of the fan radius R or main flowpassage radius R'.

The flow augmenting rings 40 thicknesses t typically should be in therange from about 0.1 to about 0.4 of the average chordal dimension C.

The ring axial spacings W should be in the range from about 5% and 50%of the average ring chord C as long as the separation between axiallyadjacent flow augmenting rings 40 is not so small that induced radialair flow through the aperture 41 is inhibited and the tip vorticescannot penetrate the axial spacings between the rings.

This means that for smaller heat sinks, having a main air flow passageof 40 mm diameter (a radius of 20 mm), the axial width W becomes agreater percentage of the average ring chord C or about 60% of C andthus preferably at least above about three mm. At smaller separations Wthe radial flow tends to be choked off and the ability of the tipvortices to penetrate into the axial spacings between the rings isreduced. In such case the advantages of the invention are diminisheduntil the spacing W becomes so small that the benefits of the inventionare suppressed. A diminishment of the benefits of the invention alsooccurs when the axial widths W between the rings 40 are made too large.

In general, a preferred and optimum geometry for an active heat sink inaccordance with the invention would involve a fan outer radius R, a mainair flow passage radius R' of about 1.03 R, a ring chord C of 0.25 R, aring thickness t of 0.3 C for small heat sinks and 0.12 C for largerunits and a ring spacing W of 0.75 C.

The aperture 41.3 is formed by spacing the back mounting plate 74 of thefan 44 away from the upper flow augmenting ring 40.3. Since the spacingW is of the order of 3 mm in a small heat sink the additional spaceoccupied by this is negligible.

With reference to FIGS. 8-11 other embodiments for the type of heat sinkas shown in FIG. 1 are illustrated. In FIGS. 8 and 9 a heat sink 80 isshown that is preferably also made with a metal extrusion process butdoes not require a floating die. Other techniques for making the heatsink 80 can be used such as by casting or with a hot wire process orassembled with previously stamped parts. The heat sink 80 has a baseplate 82 for contact with a heat generating device. Columns 84.1, 84.2,84.3 and 84.4 extend up from the base plate 82 and support a pluralityof flow augmenting rings 86 which extend radially outward in cantileverfashion parallel with the base plate 82.

When extruded the heat sink 80 has elongate walls extending across thebase plate 82 with columns 84.1, 84.3 and 84.2, 84.3 respectively inalignment with each other. Then when the main air passage 85 is cut intothe extrusion the elongate walls become heat conducting columns 84located at the periphery and end portions of the base plate 82. Thecolumns 84 are laterally spaced from each other by gaps 87 which receiveattachment clips for heat coupling to a semiconductor chip and to enableextrusion of the structure without the use of floating dies. The columns84 further are preferably placed, as measured along at least onedimension, near the central region of the base plate. The columns 84 aresufficiently thick so as to be able to transfer all the heat from thebase plate 82 to the flow augmenting rings 86.

The main air flow passage way 85 is formed in the heat sink 80 and sizedto receive a fan 88. The fins usually employed on the base plate 82 canbe dispensed with as illustrated in FIGS. 8 and 9 though they areincluded in a similar design shown in FIG. 10. Additional heat transfercapability in place of fins is provided by the lower flow augmentingring 86.1. The apertures 90 between the rings 86 are not obstructed byend located columns 84 as in the embodiment of FIGS. 1-6 so that therings extend radially outwardly in cantilever fashion from the columns84. In this manner additional radial air flow can pass through theapertures 90.

In FIG. 10 a heat sink 92 is shown similar in shape to that shown inFIG. 8 but with the lower ring segments 90.1 removed and fins 94employed on the base plate 82'. In FIG. 11 a heat sink 98 similar tothat shown in FIG. 10 is shown, but in this case additional columns 84are employed on lateral ends of the flow augmenting rings 100 similar tothe construction of the heat sink of FIG. 1.

In an alternate embodiment, similar to that shown in FIGS. 8 and 9, thecolumn pairs 84.1, 84.2 and 84.3, 84.4 are joined by metal instead of anair gap 87. This provides thicker columns 84 on respectively oppositesides of the main air flow passage 85 and assures sufficient heatconduction capability from the base plate 82 to the flow augmentingrings 86. In such case an alternate clamping device is used to hold thebase plate against a semiconductor device 32.

The dimensional relationships previously described for FIGS. 1-6 arealso applicable to the embodiments shown in FIGS. 8-11.

With reference to FIGS. 12-14 a heat sink 110 is shown wherein a fan 112is provided with a deflector 114 to pull incoming air flow from thesides and thus be less sensitive to objects located directly above theheat sink 110 over the fan 112. The heat sink, which is similar to thatas shown in FIG. 1, has flow augmenting rings 116 separated by spacessufficient to enable induced air flow as suggested by arrows 118. Themain air flow is discharged through the spaces between the fins 120.

The active heat sink 110 is shown with a deflector 114. The heatremoving capability of the heat sink is, however, not significantlydegraded when the axial intake of the fan is blocked by electroniccomponents or the like, since the availability of radial inflow of airprevents the fan from stalling.

In FIGS. 15 and 16 a heat sink 130 is shown formed of a single multifolded strip of heat conducting material 132. The strip 132 typically isfirst appropriately stamped to cut out apertures 133 that have thecross-sectional dimension of the intended main air flow passage 134 andare so located that when the strip 132 is folded to form flow augmentingrings 136 the apertures 133 align and the axial spacings W between therings 136 is as required. The strip 132 is further so shaped to providethe rectangular surface areas for the individual flow augmenting rings136 as illustrated in FIG. 16.

Initial testing of an undersized bread board type model indicates thatthe thermal resistance of a heat sink 130 comparable in size to the typeshown in and described with reference to FIG. 1 is likely to be wellbelow 0.4° C./W.

A base plate 138 is provided with suitable heat conducting columns 140.The columns 140 can be of solid metal or formed of heat pipes and serveto conduct the heat from the base plate 138 as well as support the flowaugmenting rings 136. The attachment of the columns to the rings 136 canbe by way of brazing or such other fastening technique enabling goodheat conduction from the columns 140 to the flow augmenting rings 136.The fan 142 can be attached to the structure with suitable screwsapplied to threaded holes in the columns 140 or with other techniquessuch as staking or riveting.

FIGS. 17 and 18 show an embodiment wherein a heat sink 150 is mounted sothat the fan 152 mounted thereto has its rotational axis 154 horizontalor parallel to the base plate 156 mounted to the electronic chip 158.The heat sink now is formed with a plurality of flow augmenting ringsnot all of which are within the axial span of the fan blades 160. Theflow augmenting rings are each affixed to the base plate 156 and aresupported by it so that heat conducting and ring support columns are notneeded. The last ring can be either open or closed since its axialdistance from the fan enables a sufficient amount of air to radiallyspill out from the apertures 162 between the rings. The spacings 162 atthe upper end may be open or closed.

FIGS. 19-21 show a heat sink 180 wherein a plurality of flow augmentingrings 181 attached to and supported by columns which can be eitherthermoelectric cooling columns or heat pipes 182. The cooling columns182 are essentially heat transfer devices so that the transfer of heatfrom a base plate 184 to the flow augmenting rings 181 can beefficiently implemented. The spacings and sizes of the flow augmentingrings are selected so as to obtain the turbulent induced radial air flowas shown with arrows 186.

FIGS. 22 and 23 show an extruded heat sink structure 200 that is similarto that shown for FIGS. 8 and 12. The flow augmenting rings 202,however, have differently sized central main air flow passages 204.These are selected so as present the lowest flow augmenting ring 202.1with a larger surface area since that ring is likely to require the mostcooling. Since the main air flow passage 205 is not uniformly sized asit is with the embodiments in the previous Figures, the propeller blades206 of the fan 208 are also stepped. This enables the gap S between thepropeller blades 206 and the adjacent inner edges 210 of the flowaugmenting rings to be maintained within the desired range needed todisrupt the propeller tip vortices.

FIG. 24 shows a heat sink structure 220 wherein a base plate 221 is heatconductively coupled to upper located flow augmenting rings 222 by wayof heat exchanger type fin structures 224 and the rings are in turncoupled to each other with similar fin structures 224. The orientationof the fin structures aligns air passages 226 so as to enable inducedradial inward air flow to occur as with the previously described heatsinks of this invention. The use of the fin structures 224 both enhancethe dispersion of heat from the base plate 221 as well provide aphysical support of upper located flow augmenting rings 222 without theuse of heat conducting columns. Each fin structure layer 224 can beformed of a single heat conducting sheet material that is subjected tomultiple folds in a manner that is well known in the heat exchange art.The heat exchange layers can be brazed to the flow augmenting rings orthe entire heat sink structure 220 clamped together with suitableclamps, not shown.

Having thus described several embodiments for a heat sink in accordancewith the invention, its advantages can be appreciated. Variations fromthe embodiments can be made by one skilled in the art without departingfrom the scope of the following claims.

What is claimed is:
 1. A heat sink for a heat generating device, comprising:a heat conducting body having a base plate for placement in heat conducting relationship with the heat generating device and a plurality of spaced apart heat conducting flow augmenting rings with at least one air pumping aperture between the rings; said heat conducting body having end located columns in the form of elongate substantially solid walls extending from said base plate and connected to said flow augmenting rings in heat conducting relationship; a main air flow passageway extending through said spaced apart rings and being sized to receive an axial fan with propeller blades extending towards a radially inward edge of the flow augmenting rings to deliver a main flow of air in an axial direction along said main air passageway and with a gap between the propeller blades and the inner radial edge of the rings selected to enable production of tip vortices from the propeller tips; and with ring chordal dimensions being effectively selected with respect to a radial dimension of the main air flow passageway so as to enable tip vortices from the propeller blades to be converted to useful air flow along said axial direction with a radially inwardly induced air flow between and over heat conducting flow augmenting rings that is a significant portion of the total mass flow generated along said main air flow passageway; said rings being in heat conducting relationship with said base plate so as to transfer heat from the base plate to said heat conducting flow augmenting rings; the axial width of said air pumping aperture between axially adjacent heat conducting flow augmenting rings being selected so as to enable said tip vortices to extend into the aperture and impinge upon radially inner regions of the rings and produce an annular area on at least one of said rings having a heat transfer coefficient H_(c) of at least about 75 Watts/m² /C.; so as to impart to said heat sink with said flow augmenting rings a high overall heat transfer characteristic H_(c) ; whereby the thermal resistance of said heat sink, when combined with said axial fan within the main air flow passageway, is reduced to a sufficiently low level so as to significantly enhance the removal of heat from said heat generating device.
 2. The heat sink as claimed in claim 1 wherein said rings are respectively suspended from said end located columns.
 3. The heat sink as claimed in claim 1 wherein said heat conducting body further is provided with spaced apart generally centrally located heat conducting columns extending up from said base plate, wherein each said spaced apart column is connected to a portion of said flow augmenting rings which extend outwardly from said spaced apart columns.
 4. A heat sink for a heat generating device, comprising:a heat conducting body having a base plate for placement in heat conducting relationship with the heat generating device and a plurality of spaced apart heat conducting flow augmenting rings with at least one air pumping aperture between the rings; said heat conducting body having spaced apart heat conducting columns extending up from said base plate, wherein each column is connected to a portion of said flow augmenting rings which extend outwardly from said columns in cantilever fashion; a main air flow passageway extending through said spaced apart rings and being sized to receive an axial fan with propeller blades extending towards a radially inward edge of the flow augmenting rings to deliver a main flow of air in an axial direction along said main air passageway and with a gap between the propeller blades and the inner radial edge of the rings selected to enable production of tip vortices from the propeller tips; and with ring chordal dimensions being effectively selected with respect to a radial dimension of the main air flow passageway so as to enable tip vortices from the propeller blades to be converted to useful air flow along said axial direction with a radially inwardly induced air flow between and over heat conducting flow augmenting rings that is a significant portion of the total mass flow generated along said main air flow passageway; said rings being in heat conducting relationship with said base plate through said columns so as to transfer heat from the base plate to said heat conducting flow augmenting rings; the axial width of said air pumping aperture between axially adjacent heat conducting flow augmenting rings being selected so as to enable said tip vortices to extend into the aperture and impinge upon radially inner regions of the rings and produce an annular area on at least one of said rings having a heat transfer coefficient H_(c) at least about 75 Watt/m² /C.; so as to impart to said heat sink with said flow augmenting rings a high overall heat transfer characteristic H_(c) ; whereby the thermal resistance of said heat sink, when combined with said axial fan within the main air flow passageway, is reduced to a sufficiently low level so as to significantly enhance the removal of heat from said heat generating device.
 5. The heat sink as claimed in claim 4 wherein said columns are located along at least one dimension of said base plate at a generally central region so as to enable said columns to be in proximity of heat coupled to the base plate from a heat generating device underneath a central part of the base plate.
 6. The heat sink as claimed in claim 4 wherein said flow augmenting rings and said base plate have a rectangular perimeter.
 7. The heat sink as claimed in claim 6 wherein said heat conducting body has first and second pairs of spaced apart columns adjacent the main air flow passageway and wherein said end located columns are elongate plates located along sides of the rectangular perimeter.
 8. A heat sink for heat generating devices comprising:an extruded metal heat conducting body having a base plate for placement in heat conducting relationship with the heat generating device and a plurality of spaced apart heat conducting flow augmenting rings with at least one air pumping aperture between the rings; said base plate having first and second columns extending upward therefrom respectively at different sides of the main air flow passageway and elongate substantially solid end located columns, wherein said flow augmenting rings extend in opposite directions from said first and second columns to connect with said end located columns and extend around said main air flow passageway; a main air flow passageway extending through said spaced apart rings and being sized to receive an axial fan with propeller blades extending towards a radially inward edge of the flow augmenting rings to deliver a main flow of air in an axial direction along said main air passageway and with a gap between the propeller blades and the inner radial edge of the rings selected to enable production of tip vortices from the propeller tips; and with ring chordal dimensions being effectively selected with respect to a radial dimension of the main air flow passageway so as to enable tip vortices from the propeller blades to be converted to useful air flow along said axial direction with a radially inwardly induced air flow between and over heat conducting flow augmenting rings that is a significant portion of the total mass flow generated along said main air flow passageway; said rings being in heat conducting relationship with said base plate through said columns so as to transfer heat from the base plate to said heat conducting flow augmenting rings; the axial width of said air pumping aperture between axially adjacent heat conducting flow augmenting rings being selected so as to enable said tip vortices to extend into the aperture and impinge upon radially inner regions of the rings and produce an annular area on at least one of said rings having a heat transfer coefficient H_(c) of at least about 75 Watts/m² /C.; so as to impart to said heat sink with said flow augmenting rings a high overall heat transfer characteristic H_(c) ; whereby the thermal resistance of said heat sink, when combined with said axial fan within the main air flow passageway, is reduced to a sufficiently low level so as to significantly enhance the removal of heat from said heat generating device.
 9. A heat sink for a heat generating device, comprising:a heat conducting body having a base plate for placement in heat conducting relationship with the heat generating device and a plurality of spaced apart heat conducting flow augmenting rings with at least one air pumping aperture between the rings; and undulating heat conducting elements interposed between and interconnecting adjacent rings and having passages oriented to enable said radial air flow through the passages; a main air flow passageway extending through said spaced apart rings and being sized to receive an axial fan with propeller blades extending towards a radially inward edge of the flow augmenting rings to deliver a main flow of air in an axial direction along said main air passageway and with a gap between the propeller blades and the inner radial edge of the rings selected to enable production of tip vortices from the propeller tips; and with ring chordal dimensions being effectively selected with respect to a radial dimension of the main air flow passageway so as to enable tip vortices from the propeller blades to be converted to useful air flow along said axial direction with a radially inwardly induced air flow between and over heat conducting flow augmenting rings that is a significant portion of the total mass flow generated along said main air flow passageway; said rings being in heat conducting relationship with said base plate so as to transfer heat from the base plate through said undulating heat conducting elements to said heat conducting flow augmenting rings; the axial width of said air pumping aperture between axially adjacent heat conducting flow augmenting rings being selected so as to enable said tip vortices to extend into the aperture and impinge upon radially inner regions of the rings and produce an annular area on at least one of said rings having a heat transfer coefficient H_(c) of at least about 75 Watts/m² /C.; so as to impart to said heat sink with said flow augmenting rings a high overall heat transfer characteristic H_(c) ; whereby the thermal resistance of said heat sink, when combined with said axial fan within the main air flow passageway, is reduced to a sufficiently low level so as to significantly enhance the removal of heat from said heat generating device.
 10. A heat sink for a heat generating device, comprising:a heat conducting body having a base plate for placement in heat conducting relationship with the heat generating device and a plurality of spaced apart heat conducting flow augmenting rings being formed of a multifolded heat conducting material with at least one air pumping aperture between the rings; a main air flow passageway extending through said spaced apart rings and being sized to receive an axial fan with propeller blades extending towards a radially inward edge of the flow augmenting rings to deliver a main flow of air in an axial direction along said main air passageway and with a gap between the propeller blades and the inner radial edge of the rings selected to enable production of tip vortices from the propeller tips; and with ring chordal dimensions being effectively selected with respect to a radial dimension of the main air flow passageway so as to enable tip vortices from the propeller blades to be converted to useful air flow along said axial direction with a radially inwardly induced air flow between and over heat conducting flow augmenting rings that is a significant portion of the total mass flow generated along said main air flow passageway; said rings being in heat conducting relationship with said base plate so as to transfer heat from the base plate to said heat conducting flow augmenting rings; the axial width of said air pumping aperture between axially adjacent heat conducting flow augmenting rings being selected so as to enable said tip vortices to extend into the aperture and impinge upon radially inner regions of the rings and produce an annular area on at least one of said rings having a heat transfer coefficient H_(c) of at least about 75 Watts/m² /C.; so as to impart to said heat sink with said flow augmenting rings a high overall heat transfer characteristic H_(c) ; whereby the thermal resistance of said heat sink, when combined with said axial fan within the main air flow passageway, is reduced to a sufficiently low level so as to significantly enhance the removal of heat from said heat generating device.
 11. A heat sink for removing heat from heat generating devices comprising:a heat conducting body having a base plate for placement in heat conducting relationship with the heat generating device and a plurality of spaced apart heat conducting flow augmenting rings with at least one air pumping aperture between the rings; at least one elongate column affixed to said base plate and coupled to support said flow augmenting rings above said base plate so as to transfer heat from the base plate to said heat conducting augmenting rings; said flow augmenting rings being suspended form said column in cantilever fashion; a main air flow passageway extending through said spaced apart flow augmenting rings; an axial fan sized to fit inside the main air flow passage way and having propeller blades which extend towards a radially inward edge of the flow augmenting rings to deliver a main flow of air in an axial direction along said main air passageway and with a gap between the propeller blades and the inner radial edge of the rings selected to enable production of tip vortices from the propeller tips; the axial width of said air pumping aperture between axially adjacent heat conducting flow augmenting rings being selected so as to enable said tip vortices to extend into the aperture and impinge upon radially inner portions of the rings and with ring chordal dimensions being effectively selected with respect to a radial dimension of the main air flow passageway so as to enable tip vortices from the propeller blades to be converted to useful air flow along said axial direction with a radially inwardly induced air flow between and over heat conducting flow augmenting rings so as to impart to said heat sink with said flow augmenting rings a high overall heat transfer coefficient H_(c) ; whereby the thermal resistance of said heat sink, when combined with said axial fan within the main air flow passageway, is reduced to a sufficiently low level so as to significantly enhance the removal of heat from said heat generating device.
 12. The heat sink as claimed in claim 11 wherein said heat conducting body has spaced apart heat conducting columns extending up from said base plate and wherein each column is connected to a portion of said flow augmenting rings which extend outwardly from said columns in cantilever fashion.
 13. A heat sink for heat generating devices comprising:a heat conducting body for placement in heat conducting relationship with the heat generating device and a plurality of spaced apart heat conducting flow augmenting rings with at least one air pumping aperture between the rings; a main air flow passageway extending through said spaced apart rings and being sized to receive an axial fan with propeller blades extending towards a radially inward edge of the flow augmenting rings to deliver a main flow of air in an axial direction along said main air passageway and with a gap between the propeller blades and the inner radial edge of the rings selected to enable production of tip vortices from the propeller tips; and with ring chordal dimensions being effectively selected with respect to a radial dimension of the main air flow passageway so as to enable tip vortices from the propeller blades to be converted to useful air flow along said axial direction with a radially inwardly induced air flow between and over heat conducting flow augmenting rings; a heat conducting column in the form of a heat conducting cooling pipe for heat conducting relation ship with said heat generating device and coupled to support said flow augmenting rings and transfer heat thereto; said cooling pipe comprising a thermoelectric cooling column; the axial width of said air pumping aperture between axially adjacent heat conducting flow augmenting rings being selected so as to enable said tip vortices to extend into the aperture and impinge upon radially inner regions of the rings; whereby the thermal resistance of said heat sink, when combined with said axial fan within the main air flow passageway, is reduced to a sufficiently low level so as to significantly enhance the removal of heat from said heat generating device. 